The use of the Internet continues to become an increasingly popular communication tool in business, social, and recreation activities and continues to affect how people exchange, gather, and disseminate information in their everyday lives. As the demand for faster and more efficient information and data transfer continues to increase, the development of modem technology continues to improve at a rapid pace. For example, digital subscriber line (DSL) modem systems are becoming increasingly popular.
FIG. 1 depicts a conceptual diagram of a typical prior art digital communication path using current DSL modem technology in which the principles of the present invention may be incorporated. Generally, a central site, such as an Internet service provider (ISP) 100, is digitally connected to a telephone network 120 through a DSL server modem 110. Although not shown in FIG. 1, modem 110 may include a transmitter section and receiver section resident therein. Network 120 is typically connected to a central office 130, which facilitates the transfer of data via transmission lines to a client modem 140, such as, for example, another DSL modem, which may be coupled to an end-user's personal computer (PC) 150. In turn, PC 150, before, during, or after receiving the data, can transmit data back to ISP 100 through central office 130, network 120 and modems 110 and 140. Typically, such full-duplex transmission can occur over lines of 14,000 to 16,000 feet, and often over 18,000 feet in length.
As a result of the ongoing transmit and receive signals within the communication path and within the modems, corruptive cross-talk or near-end echo is generally created whenever a portion of the transmitted signal leaks into the receive path. The leakage is typically called echo if it is due to a direct electrical connection through a hybrid circuit when a single channel (e.g., a twisted pair) is used for the transmitting and receiving paths, or is called near-end crosstalk (NEXT) if it is due to a capacitive/inductive connection between separate channels used in a dual simplex system. These undesirable echo signals produced from the transfer of data through the communication path are typically canceled by the transceiver electronics. Generally, echo signals can be adequately canceled by linear systems provided in the modems so that the receive signal can be adequately interpreted by a technique generally known as echo cancellation.
The essence of echo cancellation is to utilize a known transmission signal, apply adaptive algorithms to generate a signal representing the echo, and subtract the echo estimate from the total received signal to produce the desired signal, i.e., without the echo. To cancel the echo, the digital data being transmitted is sampled and passed through an adaptive digital echo canceler, which is typically an adaptive finite impulse response filter. The adaptive filter acts to impart the same transfer function on the transmit signal as that of the actual line load seen at the input to the receiver. Typically, this line load, for a transmission line of approximately 18,000 feet, may be 135 ohms. Thus, when the echo estimate is subtracted from the total received signal, the corruptive echo or cross-talk is typically canceled to the extent of the system's linearity and to the extent that the adaptive filter linearly matches the transmission cable characteristics.
In addition, high linearity is typically required from the receiver electronics in order to adequately quantify a signal which may be severely attenuated by the transmission cable. For example, in many cases this attenuation can amount to 40 dB of noise contribution. Therefore, because the transmit signal may be coupled into the receive signal, high linearity is also required from the transmit circuitry due to the inability of a typical linear receiver to optimally recover a signal which has been contaminated by non-linearities. Non-linearities in a communication system appear to the receiver as a noise contributor and can cause deterioration of the transmit signal, i.e., the non-linearities lower the signal-to-noise ratio (SNR) and may reduce the data rate. Thus, in order to make this technique as effective as possible, the transmit circuitry should be designed with linearity which meets or exceeds the SNR of the received signal as well as the attenuation of the transmission lines. In most high data rate applications, this linearity requirement for the transmit circuitry could exceed 70 dB or 80 dB.
FIG. 2 illustrates a portion of a server modem 200, such as a DSL modem, which includes a transmit circuitry 204 and a receiver 206. In this example, a digital signal processor (DSP) 202 provides a digital signal to transmit circuitry 204 for transmission to a user modem 220. As with many practical data communication systems, near-end echo (represented by an echo path 208) associated with a transmit signal may be present in a signal received by server modem 200. The characteristics of the near-end echo signal may be dictated by functional components in the upstream and downstream channels and/or processing performed within the telephone network, including components of transmit circuitry 204. The echo signal combines with the intended receive signal and the “corrupted” receive signal is then processed by server modem 200. An echo canceler 210 is employed by server modem 200 to compensate for the near end echo. As discussed above, in an ideal modem system, a duplicate echo signal generated by echo canceler 210 is subtracted from the signal to be received by server modem 200 to produce the desired receive signal at receiver 206. However, the sampling of the transmitted signal typically occurs before the transmit circuitry, i.e., the output signal of DSP 202 is fed into echo canceler 210. As a result, any distortions, i.e., non-linearities, introduced by the transmit circuitry will not be canceled. Thus, the linearity of the transmit circuitry must typically be on the order of linearity of the rest of the communication system components, particularly user modem receiver 220, so that the transmit circuitry's distortion does not limit the transceiver's performance. Attempts to eliminate the non-linearities by designing non-linear echo canceling filters have proven unsuccessful because it is extremely difficult to model the non-linearity present in the transmit circuitry. As such, designers have been forced to utilize costly high linearity components and accept some level of non-linearities unless the non-linearities can be designed out of the transmit circuitry.
However, it is quite difficult if not impossible to design transmit circuitry that eliminates such non-linearities. With momentary reference to FIG. 4, the transmit circuitry components typically comprise a digital-to-analog converter (DAC) 402 and a line driver or amplifier 406. Due to the power requirements typically needed by amplifier 404 to drive transmit signals through the transmission cable, which generally possesses a low impedance as reflected back to amplifier 406, large amounts of current are generally produced. The large current requirement, in turn, provides design limitations in providing a highly linear line driver or amplifier. Thus, the high linearity desired in the transmit circuitry can be compromised by the need to provide the necessary power requirements, i.e., the communication system is dominated by the line driver performance. In addition, current CMOS technology typically has great difficulty in providing line drivers to the degree of linearity required for DSL applications, particularly for newly developed HDSL2 applications.
Other known methods for attempting to reduce effects of the non-linearities introduced by the transmit circuitry include the use of an analog hybrid circuit 608 at the line driver output to compensate for the non-linearities (see FIG. 6). Generally, these hybrids provide a terminating resistor configuration, RT(H), and an impedance configuration, ZL(H), that are designed in an attempt to approximate a terminating resistor, RT 602, and a transmission line impedance, ZL 604. Although these compensating analog hybrid circuits may reduce some of the effects of the non-linearities, the analog hybrids are not readily adaptive, are not integrated into the communication device and, due to the number of additional components that are required, e.g., resistors and capacitors, can often introduce complexities in design that make the circuits undesirable from a cost, marketing, and implementation viewpoint.
Thus, a new method and apparatus for an echo cancellation scheme that compensates for the non-linearities present in transmit circuity as used in a digital communication system and overcomes the prior art is greatly needed.